Recently, from the viewpoint of conservation of the environment, various biodegradable polymers which can be decomposed in the natural environment due to the working of microorganisms present in soil or water have been developed. Among these biodegradable polymers, examples of known biodegradable polymers that can be melt-formed include: polyhydroxybutyrate; polycaprolactone; and aliphatic polyesters and polylactic acids composed of an aliphatic dicarboxylic acid component, such as succinic acid or adipic acid, and a glycol component, such as ethylene glycol or butanediol.
Among them, polylactic acid-based biodegradable polymers have excellent characteristics such as high heat resistance and high strength compared to other biodegradable polymers. Polycaprolactone-based biodegradable polymers have the characteristic that they can be molded and processed into films or plastic fibers by injection molding, extrusion molding, or melt spinning. However, these biodegradable polymers have drawbacks, for example, in shock resistance, flexibility, and rapid reduction in properties during biodegradation, and their applicable use is limited.
Therefore, attempts at blending a plurality of biodegradable polymers have been made. For example, as shown in Patent Document 1 and Patent Document 2, attempts at blending polylactic acid with various polymers have been made in order to improve the properties of polylactic acid.
However, since the use of biodegradable polymers has expanded recently, requirements therefor have also increased, and the characteristics thereof have to be improved so as to be equivalent to those of general-purpose polymers. In order to respond to these requirements, attempts at blending existing general-purpose high polymers with biodegradable polymers have been made. As a result, although biodegradability cannot be expected, the use of biodegradable polymers has made it possible mainly to reduce the used amount of general-purpose resins derived from petroleum. Thus, there is the advantage that carbonic acid gas generation and combustion heat upon disposal are lowered. Therefore, this method draws attention as a method that can reduce environmental load. For example, Patent Document 3 discloses the method of blending polylactic acid with rubber-blended shock-resistant polystyrene as a means for improving the shock resistance of polylactic acid. However, since the method described in Patent Document 3 is a method in which polylactic acid and highly shock resistant styrene are melted and mixed, and the obtained resin composition has not reached compatibilization, the improvement in toughness and heat resistance is limited.    Patent Document 1: Japanese Patent Appl. Publ. No. 2000-219803    Patent Document 2: Japanese Patent Appl. Publ. No. 9-272794    Patent Document 3: Japanese Patent Appl. Publ. No. 2005-264086